1. Field of the Invention
The present invention relates to a method of controlling the traffic intensity of radio base stations in a mobile communication system which has a plurality of mobile terminals, a plurality of radio base stations having a plurality of voice channels for radio communications with the mobile terminals in a plurality of radio service areas in which the mobile terminals are movable, and a mobile telephone switching office for governing the radio base stations to control connection of calls to and from the mobile terminals and selection of voice channels.
2. Description of the Prior Art
One conventional traffic control process for use in a cellular mobile communication system effects a quasi hand-off process, referred to as a "directed retry" process proposed by EIA (Electronic Industry Association) and TACS (Total Access Communication System), on a control channel when a new call is made while all the voice channels of a certain radio base station are in use. According to the directed retry process, when a new call is made while all the voice channels of a certain radio base station are in use, the mobile telephone switching office uses the control channel of the radio base station of which all voice channels are in use to send control channel information of nearby radio base stations which have an idle voice channel to a calling or called mobile terminal in a radio service area covered by the radio base station. Based on the control channel information supplied from the mobile telephone switching office, the mobile terminal selects a control channel with a sufficient electric field reception level from the nearby radio base stations and again attempts to make a call using the selected control channel.
For smoothly carrying out the directed retry process, there has been proposed a process for effecting call-connecting hand-off by collecting information on electric field reception levels between the radio base station and nearby radio base stations. Such a proposed process will first be described with reference to FIG. 1 of the accompanying drawings. FIG. 1 shows a general mobile communication system which comprises a mobile terminal 4, a plurality of radio base stations 2 and 3, and a mobile telephone switching office 1 for governing radio base stations 2 and 3 and connecting calls and providing communication service between subscribers and the mobile terminal 4. The radio base stations 2 and 3 can effect radio communication with the mobile terminal 4 in radio service areas A, B. A typical hand-off control process in the mobile communication system will next be described with reference to FIG. 1. The mobile telephone switching office 1 controls radio base station 2, which is providing communication service, and nearby radio base station 3 to measure the level of the electric field received from the mobile terminal 4, and collects the measured data. In the hand-off process, radio base station 2, which is providing communication service, is the original radio base station from which the call is to be rerouted or transferred, and nearby radio base station 3 is the destination radio base station to which the call is to be rerouted. If (the level of the electric field received by the destination radio base station)--(the level of the electric field received by the original radio base station) .gtoreq..DELTA., the mobile telephone switching office 1 transfers the call from the original radio base station to the destination radio base station. Value .DELTA. may be established in advance for each hand-off route in the mobile telephone switching office 1 by a radio planner depending on the geometrical conditions. For example, it is assumed that value .DELTA. is set to .DELTA.AB when a call is to be rerouted from radio service area A of radio base station 2 to radio service area B of radio base station 3, and value .DELTA. is set to .DELTA.BA when a call is to be rerouted from radio service area B of radio base station 3 to radio service area A of radio base station 2. When the mobile terminal 3 moves from radio service area A to radio service area B, a call is rerouted from radio service area A to radio service area B if (the level of the electric field received by radio base station 3)--(the level of the electric field received by radio base station 2).gtoreq..DELTA.AB. Conversely, when the mobile terminal 3 moves from radio service area B to radio service area A, a call is rerouted from radio service area B to radio service area A if (the level of the electric field received by radio base station 2)--(the level of the electric field received by radio base station 3).gtoreq..DELTA.BA.
The conventional directed retry process and hand-off process have to satisfy the conditions that the control channel of nearby radio base stations other than the radio base station for which all voice channels are in use are received at a sufficient electric field level and that there is also an idle voice channel available. Consequently, the unwanted delay required from the time when a call is made to the time when the call is actually connected, i.e., the time when a call is established in a voice channel, may be lengthy depending on the number of attempts made to connect the call.